An antenna is used to capture electromagnetic energy when operating in a receive mode, and to radiate such energy when in a transmitting mode. Accordingly, an antenna is a typical part of a communication system that also includes a transmitter and receiver, for example. To increase the antenna aperture, one or more reflectors may be arranged adjacent an antenna feed. An array feed including multiple elements may be used with such a reflector system to provide multiple beams or electronic scan capability.
U.S. Pat. No. 6,236,375 to Chandler et al. discloses a reflector antenna including a feed array, a subreflector, and a main reflector, which are oriented to define an offset Gregorian antenna geometry. The antenna feed includes a plurality of separate feeds that are aligned on a predetermined contour and connected to a feed network to produce a plurality of composite illumination beams. The subreflector and main reflector are positioned so that the focal point of the main reflector is approximately coincident with the focal point associated with the convex side of the subreflector. The feed is positioned in proximity of the focal point associated with the concave side of the subreflector.
U.S. Pat. No. 4,203,105 to Dragone et al. discloses a feed array aligned with a confocal reflector system that includes a subreflector aligned with a main reflector at a coincident focal point. U.S. Patent Application Publication No. 2004/0008148 to Lyerly et al. also discloses a Gregorian antenna reflector system including a feed array, a subreflector, a main reflector, and at least one other subreflector.
U.S. Pat. No. 6,424,310 to Broas et al. discloses a feed array, a subreflector, and a main reflector, which are oriented to define a dual offset Cassegrain antenna geometry. The coincidental focal points of the main reflector and the subreflector are located on the convex side of the subreflector.
U.S. Pat. No. 6,215,452 to Chandler et al. discloses a feed array, a subreflector, and a main reflector, which are oriented to define a front-fed dual reflector antenna geometry. The coincident focal points of the main reflector and the subreflector are located on the concave side of the subreflector.
U.S. Pat. No. 6,211,835 to Peebles et al. discloses a feed array, a subreflector, and a main reflector, which are oriented to define a side-fed dual reflector antenna geometry. The coincidental focal points of the main reflector and the subreflector are located on the convex side of the subreflector.
A prior art antenna system 20 is now described with reference to FIG. 1. This antenna system 20 includes a feed array 22 that illuminates a subreflector 23 that, in turn, reflects the energy to a main reflector 21. The subreflector 23 is aligned with the main reflector 21 at a coincident focal point 24 in what is termed a near field Gregorian configuration. Unfortunately, such a system 20 may be mechanically complex due to the relatively large displacement required between the main reflector 21 and the subreflector 23.
There are two fundamental approaches for an array fed multiple beam or electronically scanned antenna system. The first is the Gregorian configuration as disclosed in U.S. Pat. Nos. 6,236,375 and 4,203,105. The second is a focused system like the Cassegrain systems of U.S. Pat. Nos. 6,424,310; 6,215,452; and 6,211,835.
The focused system uses a focused antenna where the reflector(s) serves to focus the energy incident on the main reflector at a single point. Most reflector antennas are focused systems that use a single feed aligned to the focal point of the reflector or reflector system. When an array feed is used with a focused reflector system, feed array elements that are not on the focal point produce beams that have significant phase error, since they are not focused, resulting in distorted beam shapes and reduced beam gain. Multiple elements can be combined to overcome some of these effects, but the fundamental effect of pattern degradation as the beams are steered away from broadside is still present.
Another technique is to use a very long focal length to reduce the defocusing effects with scan. In this technique, the feed element displacement from the focal point required to scan the beam is proportional to the focal length. As a result, for a given beam displacement range the feeds have to increase in size and number of elements as the focal length grows. Another fundamental aspect of a focused system is that the beams are scanned primarily by using different feed elements so that any particular beam may only use a small fraction of the feed. Consequently, such a focused system has a low feed utilization.
The Gregorian or confocal (focal point of main and subreflector are coincident) dual reflector arrangement is distinctly different from the focused reflector systems. The optics of a Gregorian system concentrate the energy incident on the main reflector to a smaller aperture rather than a focal point. This property is sometimes referred to as aperture magnification since a scaled replica of the fields incident on the main reflector are produced at the feed. As a result, the Gregorian system may overcome many of the shortcomings of a conventional focused system because there is reduced beam distortion and most of the feed is utilized.
The drawback with a Gregorian system is the large and cumbersome geometries that are required. The magnification is proportional to the ratio of the focal lengths. Consequently, to use a small feed and produce a large aperture with minimal blockage, a relatively large subreflector with significant separation from the main reflector is required.